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WEEK 19:
Drug Interactions:
| Question | Answer |
|---|---|
| difference between haemostasis and thrombosis | haemostasis is the process of preventing blood loss and thrombosis is the actual formation of a thrombus to clot the vessel |
| coagulation cascade | complex series of proteolytic enzymes and cofactors leading to the formation of insoluble fibrin from soluble fibrinogen |
| heparin types | unfractionated heparin and LMWH |
| unfractionated heparin | short duration of action |
| LMWH | fragments of heparin and have a longer duration of action so more predictable pharmacokinetics and can be given subcutaneously (dose based on weight) once a day post surgery without monitoring to treat DVT/PE |
| thrombin IIa | converts fibrinogen to fibrin and activates factor XIII which stabilises fibrin links |
| factor XIII | stabilises fibrin links |
| antithrombin III inhibitor | neutralises all serine proteases in the cascade |
| heparin mechanism of action | binds to ATIII which increases ATIII's ability to inactivate thrombin IIa |
| antithrombin III | inactivates thrombin IIa |
| how does LMWH speeds up anticoagulatory effects | speeds up anticoagulatory effects of ATIII on factor Xa by binding to ATIII |
| suffix of heparin examples | in/ arin |
| warfarin | oral anticoagulant - reversible vitamin K antagonist |
| warfarin mechanism of action | inhibits VKOR so vitamin K cannot be activated so vitamin K dependent y-glutamyl carboxylase cannot activate clotting factors (II, VII, IX, and X) |
| how long does warfarin take to work and why | several day because half life of clotting factors (clotting factors remain in the blood and need to break down) |
| how do you reverse the actions of warfarin | give vitamin K |
| what makes VKOR | vitamin K epoxide |
| interaction definition | occurs when effect of one drug are changed by the presence of another drug, food, drink or environmental chemical agent |
| how can chances of interactions be increased (4) | polypharmacy (regular use of several medications), conditions eg renal impairment, drugs with narrow therapeutic window, and food |
| BNF | lists interactions and classifies them into severe, moderate, and mild |
| what can influence pharmacokinetic mechanisms of interaction (3) | absorption (two drugs interacting alter rate of uptake), pH, and binding of drugs (so take drugs at different times) |
| MDR1 | multiple drug resistance transporter/ P-glycoprotein/ ATP binding cassette (ABC) - may be induced or inhibited |
| digoxin | MDR1 substrate and induction of MDR1 reduces bioavailability by increasing efflux used to treat heart failure and arrythmias |
| what induces MDR1 | rifampicin |
| what inhibits MDR1 | verapamil |
| how does pretreatment with rifampicin affect digoxin absorption | pretreatment with rifampicin for several days increases efflux process from intestinal epithelial cell to lumen and therefore decreases absorption of digoxin |
| explain displacement | acidic drugs bind to albumin in the blood and displace protein bound drug so free drug is left in the blood but too much can lead to toxicity |
| phase 1 reactions in the liver | makes molecules more reactive via oxidation, reduction and hydrolysis |
| phase 2 reactions in liver | combine with other molecules to convert it into inactive form so it can be excreted eg glucuronic acid |
| danger of enzyme inhibition | effects occur immediately and increases as dose increases so can lead to toxicity easily |
| drugs with a narrow therapeutic range (4) | warfarin, theophylline, digoxin, and phenytoin |
| cytochrome P450 inhibition | prevents drugs being broken down in liver with a rapid onset of 1-2 days but often reverse quickly when inhibitors are stopped being given |
| examples of cytochrome P450 inhibitors | H2 receptor antagonists, antifungal agents, and macrolides |
| dangers of cytochrome P450 inhibition and warfarin | CYP450 usually breaks down warfarin but the CY450 inhibitor mean INR risk increases as warfarin isnt cleared from the blood leading to increased risk of bleeding |
| INR | international normalised ratio (how long it takes for blood to clot) |
| BNF of warfarin and macrolides | severe so INR should be monitored and reduce dose (if appropriate) to maintain INR and prevent bleeding |
| BNF of warfarin and quinolone antibiotics eg ciprofloxacin | severe and may interfere with CYP450 dependent metabolism and may alter gut flora and reduce vitamin K levels so patient is more likely to bleed |
| BNF macrolides and simvastatin | severe- contraindicated as it increases side effects |
| interaction between amlodipine and statin | 20mg simvastatin (max dose) |
| why is atorvastatin used instead of simvastatin when interacting with amlodipine | risk is much lower in atorvastatin |
| enzyme induction | increase activity of metabolising enzymes eg rifampicin and carbamazepine |
| effect of enzyme induction on warfarin | breaks down warfarin in a week or 2 and may persist when enzyme inducer is stopped being given |
| interaction between aspirin and warfarin | enhance bleeding effects |
| NSAIDs | non steroidal anti-inflammatory drugs |
| why is NSAIDs never used with warfarin | associated with gastric bleeding |
| why is clopidogrel and omeprazole/ esomeprazole (PPIs) interaction not used | omeprazole makes clopidogrel less effective because they are both biotransformed by the same CYP450 (CYP2C19 and CYP3A4) so clopidogrel isnt converted into active metabolite |
| what is used instead of omeprazole/ esomeprazole (PPIs) with clopidogrel | pantoprazole as it does not affect the same CYP450 needed to convert clopidogrel into an active metabolite |
| types of pharmacodynamic interactions | additive/ synergistic and antagonistic |
| pharmacodynamic interactions | effects of one drug are changed by presence of another drug at site of action |
Created by:
kablooey
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